There is known a conventional optical modulation system comprising a light source, an optical device supplied with a light beam from the light source, and an electric field applying device for applying an electric field to the optical device. The optical device comprises a substrate, an incident optical waveguide formed on the substrate for receiving the light beam incident thereto, two phase-shift optical waveguides formed on the substrate to be branched from the incident optical waveguide for varying a phase of a transmitted light beam in response to an electric field intensity, and an outgoing optical waveguide formed on the substrate to join the phase-shift optical waveguides.
In the conventional optical modulation system, however, a phase difference is caused between the branched optical waveguides even if the intensity of the applied electric field is equal to zero, depending upon manufacturing conditions of the optical waveguides. This results in fluctuation of a ratio of the variation of light intensity relative to the variation of the applied electric field, namely, fluctuation of an optical modulation efficiency.
In addition, the conventional optical modulation system is disadvantageous in that the optical device tends to deteriorate when the magnitude of the applied electric field is great.
It is an object of this invention to provide an optical modulation system capable of readily adjusting a phase difference when the intensity of an applied electric field is equal to zero.
It is an object of this invention to provide an optical modulation device capable of avoiding deterioration of an optical device even if the magnitude of an applied electric field is great.